This invention is related to the field of digital image signal processing, and more particularly to a system for detecting video signal image intensity gradient such as a luminance gradient associated with image fading, e.g., from a normal image to black level or vice-versa.
A video sequence such as represented by a television signal, for example, is a series of motionless images shown in rapid succession to give a viewer an impression of continuous motion. Each frame conveys distinctive image information, and the high frame rate necessary to achieve an appearance of motion often results in a significant amount of redundant temporal information among adjacent frames. Motion compensation coding, a form of data compression, is a form of predictive coding in a temporal dimension that is often used to attempt to remove such temporal redundancy.
In the absence of a scene change from one frame to the next, image motion from one frame to the next accounts for much of the variation in image intensity from one frame to the next. In motion compensated predictive image coding, the current frame is predicted from a previously coded frame by estimating the motion between the two frames and compensating for the motion. The difference between the current frame and the prediction of the current frame is commonly called the (motion compensated) residual image, which is coded. The energy in the residual image is typically much less than the energy in the original image due to the removal of the redundant temporal information. Encoding the residual information rather than the original image information results in a more efficient use of the data bits available for coding.
Motion compensated predictive coding may assume many forms. One popular approach is based on block matching. In this approach the current image frame is partitioned into a prescribed number of rectangular regions or blocks, and a search is performed for the displacement in an adjacent frame which produces the best match among possible blocks in the adjacent frame. A motion vector with associated x,y coordinates establishes the relationship between the block in the current frame and its best match in the adjacent frame. Motion compensated (residual image) coding is an example of inter-frame (flame-to-frame) coding. In cases where motion compensated residual coding does not produce acceptable results (eg., where the prediction is not good as when a scene changes from one frame to the next), better results may be obtained by intraframe coding, where the video information of the frame is itself coded without motion compensation.
Various types of image coding/compression, including interframe predictive motion compensated coding as discussed above, are discussed by Ang et al. in Video Compression Makes Big Gains, IEEE Spectrum, October 1991, for example. In particular, this article describes a CCITT H.261 video motion coder including provision for intra-frame coding, and inter-frame predictive motion compensated residual coding compatible with the proposed MPEG (Moving Pictures Expert Group) image coding standard (ISO/IEC 13818-2, November 1993). The proposed MPEG standard also employs a motion compression algorithm with both inter- and intra-frame modes.
Under certain conditions some motion estimators are not sophisticated or efficient enough to eliminate image "blockiness" artifacts (a noticeable difference in detail among pixel blocks that constitute an image). Such an artifact may be produced, for example, by motion estimators employing Mean Square Error or Mean Absolute Error processing, due to an erroneous match. More specifically, during an image fade from a normal image to black or vice-versa, frame-to-frame changes in image texture (detail) cannot be accurately tracked by other than highly sophisticated motion estimators. This inability to track frame-to-frame image detail leads to luminance tracking instead. Luminance tracking produces wrong information from the motion estimator, since if the image is motionless a luminance change with fading will falsely suggest motion. In other words, a frame-to-frame luminance gradient associated with image fading may fool the motion estimator into thinking there is motion when the motion estimator cannot discern that the image detail has not changed. Such tracking of a luminance gradient produces random, unpredictable results (eg., false "motion vectors") which are essentially useless and detrimental to coding efficiency.